Conventionally, a coherent integration technique is well known in this type of target observation devices. As described in non-patent document 1, for example, the coherent integration improves a signal-to-noise ratio (SNR) by averaging observation values acquired from the reflected signal from the target.
The coherent integration provides signal processing using the statistical nature as follows. It is supposed that the independent identical distribution characterizes observation values. Averaging these values for the number of times assumed to be NUMCI multiplies the SNR as an output value after the averaging by NUMCI according to the central limit theorem. In this case, the SNR between an input signal and an output value is equivalent to a process gain that is multiplied by NUMCI as follows.Process gain=(Output SNR)/(Input SNR)    Non-patent Document 1: Mahafza, Bassem R. 2005. Radar Systems Analysis and Design Using MATLAB(R). 2d ed. Chapman & Hall/CRC, Taylor & Francis Group, ISBN-10: 1-58448-532-7, ISBN-13: 978-1-58488-532-0: 4.4.1-4.4.2
The technique provides an excellent result when there is no relative speed between the observation device and a target. When there is any relative speed between the observation device and a target, however, performing the coherent integration on a reflected wave from the target degrades a signal component due to an effect of the relative speed. The process gain degrades compared to a motionless target.